Climate warming is pushing aquaculture species to their physiological limits through elevated temperature pressures impacting feed intake, nutrient use, growth, health, and welfare. Emerging research shows that diet (and dietary supplements) can play a significant role in modulating the stress response and improving thermal tolerance, which is pivotal in aquaculture species as the oceans continue to warm. 5-aminolevulinic acid (5-ALA), is an emerging dietary supplement that is the sole precursor to heme and fundamental for the formation of hemoproteins involved in aerobic energy metabolism (haemoglobin, myoglobin, cytochrome c oxidase) and the antioxidant system (catalase). Using juvenile barramundi as a model species, a feed trial was conducted for 6 weeks utilising 5-ALA supplementation in aquafeeds at 3 inclusion levels (control (0 ppm), 15 ppm, 30 ppm). Water temperature was ramped up from 30°C to 37°C, over 4 weeks to determine the diet effects under chronic thermal stress conditions. Although haemoglobin levels and catalase activities remained similar among treatments, growth rates increased with 5-ALA supplementation. 5-ALA supplementation also increased mitochondrial respiration in Oxidative Phosphorylation (OXP) with substrates supporting Complexes I and II. However, oxygen flux in the Leak (non-phosphorylating) respiration state remained similar, indicating that the Leak/OXP ratio decreased with 5-ALA. Therefore, the capacity and apparent efficiency to synthesise ATP had increases with 5-ALA. Overall, as a dietary supplement 5-ALA holds promise in aquafeeds through increasing mitochondrial efficiency, which may account for the observed increases in growth rates, which can improve production efficiency during periods of environmental stress within the aquaculture industry.